Plant-pest interactions play an important role in shaping both natural and agricultural ecosystems. Crop losses due to pest attack remain a serious problem in the U.S., despite intensified use of chemical pesticides. As an example, late blight (Phytophthora infestans) is one of the most important diseases of potato (Solanum tuberosum L.) worldwide causing both the destruction of plants in the field and rotting of tubers during storage. Currently, there are no late blight resistant potato cultivars that meet US commercial standards (Douches et al., Amer. Potato J. 74: 75–86 [1997]). Although late blight has been effectively controlled for many years with both protective and systemic fungicides, more aggressive, fungicide resistant genotypes of P. infestans now predominate in many parts of North America making control more difficult (Deahl et al., Phytophthora infestans 150. Boole Press Ltd., pp 362 [1995]; Goodwin et al., Phytopathology 85: 473–479 [1995]). In fact, it is estimated that P. infestans cost US growers $155 million in 1999 (Sender, Agricultural Genomics 3: 4–6 [2000]). Additional problems associated with the use of chemical pesticides, which include environmental contamination, increase production costs, and threats to human health, have resulted in increasing demand for decreased use of chemical pesticide.
Therefore, an attractive alternative to pesticides, and possible resolution of the environmental and economic problems they pose, is the development of crop protection strategies that maximize the plant's “built-in” defense capabilities, or phytochemical defense mechanisms. One set of such defense mechanisms, as revealed by recent evidence, is the synthesis of oxylipins (oxygenated fatty acids), which are a critical component of both constitutive and induced mechanisms for plant defense against a broad spectrum of pests (Farmer and Ryan, Proc. Natl Acad. Sci. USA 87: 7713–7718 [1990]; Farmer and Ryan, Plant Cell 4: 29–134 [1992]).
Oxylipin metabolism in plants, although complex in its details, can be viewed as a simple two-step process. The first step involves the addition of molecular oxygen to polyunsaturated fatty acids such as linoleic acid and linolenic acid. This reaction is performed enzymatically by lipoxygenase (LOX). Plant LOXs catalyze the stereospecific addition of O2 to either the 9 or the 13 position of C18 fatty acids, thus generating 9- or 13-fatty acid hydroperoxides, respectively. Fatty acid hydroperoxides are also generated non-enzymatically by free radical-catalyzed lipid peroxidation; this route of hydroperoxide formation is likely to be an important part of the hypersensitive response of plants to avirulent pathogens.
The second phase of oxylipin metabolism involves the metabolism of fatty acid hydroperoxide intermediates to different classes of bioactive oxylipins. Four major sub-branches of fatty acid hydroperoxide metabolism have been described, all of which convert 13-hydroperoxy linolenic (and in some cases other hydroperoxides) to defense-related compounds. Of these four branches, the most well known is the synthesis of jasmonic acid and related cyclopenta(e)ones. Much less is known about another branch, which is the synthesis of divinyl ether fatty acids.